This invention relates to the general subject of seismic exploration and, in particular, to seismic interpretation and to methods for improving the quality of picked and interpreted seismic data.
The broad goal of a seismic survey is to image or map the subsurface of the earth by sending energy down into the ground and recording the xe2x80x9cechoesxe2x80x9d that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is systematically positioned at a variety of locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth, is partially reflected, and, upon its return, is recorded at a great many locations on the surface. The seismic signals are partially reflected from discontinuities of various types in the subsurface (including reflections from xe2x80x9crock layerxe2x80x9d boundaries) and the reflected energy is transmitted back to the surface of the earth where it is recorded as a function of travel time. The sensors that are used to detect the returning seismic energy are usually geophones (land surveys) or hydrophones (marine surveys). The recorded returning signals, which are at least initially continuous electrical analog signals which represent amplitude versus time, are generally quantized and recorded as a function of time using digital electronic so that each data sample point may be operated on individually thereafter.
Multiple source activation/recording combinations are subsequently combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2D) seismic survey, the recording locations are generally laid out along a single straight line, whereas in a three dimensional (3D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3D survey produces a data xe2x80x9ccubexe2x80x9d or volume that is, at least conceptually, a 3D picture of the subsurface that lies beneath the survey area. In reality, though, both 2D and 3D surveys interrogate some volume of earth lying beneath the area covered by the survey.
A seismic survey is composed of a very large number of individual seismic recordings or traces. In a typical 2D survey, there will usually be several tens of thousands of traces, whereas in a 3D survey the number of individual traces may run into the multiple millions of traces. Chapter 1, pages 9-89, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, contains general information relating to conventional 2D processing and that disclosure is incorporated herein by reference. General background information pertaining to 3D data acquisition and processing may be found in Chapter 6, pages 384-427, of Yilmaz, the disclosure of which is also incorporated herein by reference.
A modern seismic trace is a digital recording (analog recordings were used in the past) of the acoustic energy that has been reflected from inhomogeneities or discontinuities in the subsurface, a partial reflection occurring each time there is a change in the acoustic properties of the subsurface materials. The digital samples that make up the recording are usually acquired at 0.002 second (2 millisecond or xe2x80x9cmsxe2x80x9d) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Each discrete sample in a conventional digital seismic trace is associated with a travel time, and in the case of reflected energy, a two-way travel time from the source to the reflector and back to the surface again, assuming, of course, that the source and receiver are both located on the surface. Many variations of the conventional source-receiver arrangement are used in practice, e.g. VSP (vertical seismic profiles) surveys, ocean bottom surveys, etc. Further, the surface location of every receiver in a seismic survey is carefully tracked and is generally made a part of the recorded trace (as part of the trace header information). This allows the seismic information contained within the traces to be later correlated with specific surface and subsurface locations, thereby providing a means for posting and contouring seismic dataxe2x80x94and attributes extracted therefromxe2x80x94on a map (i.e., xe2x80x9cmappingxe2x80x9d).
The data in a 3D survey are amenable to viewing in a number of different ways. First, horizontal xe2x80x9cconstant time slicesxe2x80x9d may be extracted from a stacked or unstacked seismic volume by collecting all of the digital samples that occur at the same travel time. This operation results in a horizontal 2D plane of seismic data. By animating a series of 2D planes it is possible for the interpreter to pan through the volume, giving the impression that successive layers are being stripped away so that the information that lies underneath may be observed. Similarly, a vertical plane of seismic data may be taken at an arbitrary azimuth through the 3D volume by collecting and displaying the seismic traces that lie along the path of selected azimuth. This operation, in effect, extracts an individual 2D seismic line from within the 3D data volume.
Seismic data that have been properly acquired and processed can provide a wealth of information to the explorationist, who is one of the individuals within an oil company whose job it is to identify potential drilling sites. For example, a seismic profile gives the explorationist a broad view of the subsurface structure of the rock layers and often reveals important features associated with the entrapment and storage of hydrocarbons such as faults, folds, anticlines, unconformities, and sub-surface salt domes and reefs, among many others. During the computer processing of the seismic survey data, estimates of subsurface rock velocities are routinely generated and near surface inhomogeneities are detected and displayed. In some cases, seismic data can be used to directly estimate rock porosity, water saturation, and hydrocarbon content. Less obviously, seismic waveform attributes such as phase, peak amplitude, peak-to-trough ratio, and a host of others, can often be empirically correlated with known hydrocarbon occurrences and that correlation applied to seismic data collected over new exploration targets.
Of course, the positioning of a drilling site is often critically dependent on the seismic data as interpreted by the explorationist, with the positioning largely determining the success or failure of the venture. An integral element of the process of seismic interpretation is the creation of a map that shows the lateral extent and depth (or time) of one or more target horizons. Although this map might be assembled in many ways, in a typical case the explorationist uses both printed and computer-displayed seismic records to trace the occurrence of specific seismic reflectors and/or seismic features throughout the survey, these reflectors and/or features being ones that are associated with a subsurface rock unit of interest. The general process of identification and selection of seismic events throughout a seismic section or volume is known as xe2x80x9cpickingxe2x80x9d to those skilled in the arts.
Operationally, the explorationist usually begins the process of interpretation by locating the reflector of interest on seismic traces near a location where there is substantial confidence that it can be found and accurately characterized in the seismic data. For example, seismic traces that have been collected near an existing well are good candidates for use as a starting point, because the location of the target subsurface unit can often be verified via the use of synthetic seismograms that have been calculated from well logs that were taken in the well. In other cases, the explorationist might xe2x80x9ctiexe2x80x9d an unpicked seismic line with a picked seismic line that crosses it, etc. All of these methods are well known to those in the seismic interpretation arts.
However, the difficulty arises when the explorationist attempts to extend his or her interpretation away from those seismic traces wherein the time-location of the horizon of interest is known with some degree of certainty. Although the seismic reflector character might be known at a particular location, away from that location the reflection character is normally expected to vary due to, for example, changing rock unit thickness and composition. Further, the target reflector might not be laterally continuous throughout the survey region because of the presence of faults, pinch outs, truncations, etc. However, the explorationist will typically be charged with finding all of the occurrences of the reflector of interest wherever that unit occurs. Thus, the explorationist will need to laboriously examine the entire seismic data set in order to follow the reflector(s) of interest wherever they might go in time or space.
Of course, the explorationist/interpreter is not without tools to help him or her complete this undertaking. There is a bewildering array of automated picking algorithms which are designed to assist the user by selecting additional traces based on the explorationist""s pick. These so-called xe2x80x9cseededxe2x80x9d methods move from the xe2x80x9cknownxe2x80x9d to the xe2x80x9cunknownxe2x80x9d, in that they start from a xe2x80x9cseedxe2x80x9d or initial reflector pick from the interpreter, take that pick as correct, and then locate the chosen event on adjacent traces.
Predictably, where the reflection character changes seeded methods will often either jump to the wrong event or decide that the tracked reflector has ended. In either case, the interpreter will then need to determine the point at which the automated picking algorithm has gone astray and then provide another pick so that the algorithm can extend the corrected pick further onward. This sort of interaction between the explorationist and the software can be tedious and time consuming.
Heretofore, as is well known in the seismic processing and seismic interpretation arts, there has been a need for a method of improving the quality and efficiency of seismic event picking which does not require seeding by the interpreter. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.
In accordance with the present invention, a method of seismic interpretation is disclosed hereinafter that is designed for use in improving the quality and speed with which an explorationist can pick and interpret a seismic volume. The instant invention is most suitable for application to seismic data after final migration and stack. It is further most preferably applied to 3-D data, although it could also be applied to grids of 2-D seismic data or even a single 2-D seismic line if that were desired.
In brief, the instant inventors have discovered a method of unseeded picking that operates on an entire seismic volumexe2x80x94or on a sub-volume specified by the userxe2x80x94to create a collection of patches or areas of similar-character reflector picks. The method is unsupervised initially and builds an output database that contains the time and amplitude of each xe2x80x9cassignedxe2x80x9d event, an assigned event being a seismic reflection that is determined to be similar in some sense to its neighboring traces. In principle, the database of events taken together represent a complete pickingxe2x80x94but not an interpretationxe2x80x94of the seismic data.
As a next step, the interpreter accesses the seismic data from which the events database was compiled. The interpreter makes an initial pick of a reflector of interest, which pick is compared against the database of assigned events. If the picked event may be found within the database as an assigned event, all of the picks associated with that particular assigned event (the patch) are read and automatically posted on the interpreter""s section or volume. The interpreter then moves to the outer limit of the newly-picked traces and picks another trace, which pick is again compared with the database and, if it corresponds to another assigned event, those picks are read and automatically posted on the section. In this manner, the user may quickly move through the seismic volume and produce a completely picked section in a minimal amount of time.
The foregoing has outlined in broad terms the more important features of the invention disclosed herein so that the detailed description that follows may be more clearly understood, and so that the contribution of the instant inventors to the art may be better appreciated. The instant invention is not to be limited in its application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. Rather, the invention is capable of other embodiments and of being practiced and carried out in various other ways not specifically enumerated herein. Finally, it should be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting, unless the specification specifically so limits the invention.